Plasma stabilization by rotation of arc discharge tube



Feb. 4, 1969 SIMON ET AL 3,426,233

PLASMA STABILIZATION BY ROTATION OF ARC DISCHARGE TUBE Filed Dec. 1:, 1965 Sheet of 5 STORED ENERGY F DISCHARGE souace HV 1 TRIGGER ClRCUlT--- SWITCH 4 RIGHT SERVICE ASSEMBLY SERVICE BLY MOTOR GAS 98 94 0.0.

0.0. IONIZING ENERGY IZING INVENTORS. KENNETH C. ROGERS, DONALD R. SlMON a [6 FRANCIS R. SILEO ward-n74 771a.

We vbmwiol their ATTORNEYS Feb 4,1963. 1 I MO ETAL 3,426,233

I PLASMA STABILIZATION BY ROTATION OF ARC DISCHARGE TUBE med D 5, 15, 1935 Sheet of 3 Q 2-6 5/7514 58 /-Z 6/60 527 ,,fl',.' |i 5 I I 65 74 INVENTORS. KENNETH G. ROGERS, DONALD R. SIMON 8| FRANCIS R. SILEO Feb. 4, 1969 R $|M0N ET AL 3,426,233

PLASMA STABILIZATION BY ROTATION OF ARC DISCHARGE TUBE Filed Dec. 13, 1965 Sheet 3 of 5 FIG 6 Power Curve Arc Plasma Column Single Turn Theta Pinch Coil 3 v F/G'. 7

r=R v +26 i BO r O INVENTORS. KENNETH 0. ROGERS,

DONALD R. SIMON a FRANCIS R. SILEO Wu, Mm

Sheath wear ATTORNEYS United States Patent 3,426,233 PLASMA STABILIZATION BY ROTATION 0]? ARC DISCHARGE TUBE Donald R. Simon, Bloomingdale, NJ., and Kenneth C. Rogers, New York, and Francis R. Sileo, Saratoga Springs, N.Y., assignors to Vitro 'Corporation of America, New York, N.Y., a corporation of Delaware Filed Dec. 13, 1965, Ser. No. 513,447 US. Cl. 313161 Int. Cl. H01 1/50, 3/32, 29/76 19 Claims ABSTRACT OF THE DISCLOSURE This invention relates to methods and apparatus which employ a high density plasma to provide useful effects as, for example, an output of radiation in the form of X-rays or ultraviolet rays of short wavelength. In a more limited aspect, this invention relates to methods and apparatus wherein radiation is obtained from a high density plasma by subjecting it to a magnetic compression effect.

A well known way of creating a plasma is to develop in a gaseous medium between two spaced electrodes an electrical discharge which ionizes the medium. In devices such as fluorescent tubes wherein the discharge takes place through a gaseous medium which is of low density because it is under low pressure, the plasma column is stable in its position between the two electrodes. As, however, the density of the medium increases and the electrical discharge accordingly becomes an arc type discharge, the heat from the arc causes a buoyancy distortion in the path followed by the arc plasma column. That is, as exemplified by the usual are developed between widely spaced electrodes in air under atmospheric pressure, the path of the arc varies with time in a random wavy manner so as to be characterized over its length by shifting perturbations or kinks. In general, the higher the density of the medium in which the arc is produced, the-more pronounced those perturbations become.

In accordance with one aspect of the invention, we have discovered that an arc plasma column in a high density gaseous medium may be stabilized in position by preliminarily imparting to the gaseous fluid a mechanically induced spin or rotation around the axis of spacing of the electrodes producing the arc. As later explained in detail, such spinning of the fluid stabilizes the position of the arc plasma column by a pressure gradient efiect which varies directly with the density of the fluid so that, the higher such density, the greater the stabilizing influence of the spin on the column.

In accordance with another aspect of the invention, we subject a high density plasma to a magnetic compression effect to produce emanation from the plasma of high energy radiation such as soft X-rays or ultraviolet rays of short wavelength. The magnetic compression technique has been used heretofore in experimental devices aiming at the attainment of self-sustaining controlled nuclear fusion. In those prior art devices, howice ever, the object has been to minimize emission of radiation from the magnetically bottled plasma in order the better to reach the critical temperature necessary for a nuclear fusion reaction to be initiated or to be self-sustaining. In contrast, in a system according to our invention, the emission of radiation from the magnetically compressed plasma is deliberately maximized. Further, in those prior art devices, the density of the gaseous medium has been on the order of 2X10 kilograms/meter maximum, whereas in our system the density of the gaseous medium is at least 1000 times greater. Still, further, in the mentioned prior are devices, the time from start to finish of the dynamic reaction produced in the device has been on the order of 1000 microseconds or greater, whereas in our system the dynamic reaction which produces the desired radiation has a start to finish time of the order of one microsecond.

It is further in accordance with our invention to stabilize an arc plasma column in a high density gaseous medium by the mentioned pressure gradient effect and to then apply a magnetic compression to the stabilized column so as to cause emission of high energy radiation.

For a better understanding of the invention, reference is made to the following description of representative embodiments thereof and to the accompanying drawings wherein:

FIG. 1 is a plan view (partly schematic and partly in cross-section) of apparatus according to the invention; the cross-section being taken as indicated by the arrows 1--1 in FIG. 4;

FIG. 2 is an enlarged plan view (in cross section) of the central poition of the plasma tube of the FIG. 1 apparatus;

FIG. 3 is an enlarged front elevation view in cross section of the right service assembly of the apparatus of FIG. 1;

FIG. 4 is a side elevation in cross section of the FIG. 1 apparatus, the cross section being taken indicated by the arrows 44 in FIG. 1; and

FIGS. 59 inclusive are schematic diagrams explanatory of the mode of operation of the FIG. 1 apparatus.

Apparatus Referring now to FIGS. 1 and 2, a hollow transparent quartz container cylinder 15 (of about 1" outer diameter and 36" length) is filled with a high density ionizable gaseous medium 16, e.g., pure xenon at an absolute pressure of one atmosphere. At its left and right ends, cylinder 15 is supported by, respectively, a left service assembly 17 and a right service assembly 18, The two assemblies are constructed to permit spinning of the cylinder about its own axis.

A thoriated tungsten electrode rod 19 (of which the center portion is shown broken away in FIG. 1) extends from unit 17 through the interior of cylinder 15 to a blunt termination forming an anode electrode 20 within the cylinder. Another thoriated tungsten electrode rod 21 (of which the center portion is broken away in FIG. 1) extends through the interior of the cylinder to a pointed termination 22 providing within the cylinder a cathode electrode spaced from anode 20 by an axial gap 24 disposed at a central position in the length of the container cylinder. The two electrode rods and the electrodes thereon are all coaxial with the axis of cylinder 15 and (as later described) are maintained by units 17 and 18 in rotatably fixed relation with cylinder 15 to spin therewith. The elements 15, 16 and 19-24 together form a plasma tube designated generally by the reference numeral 25.

Tube 25 is rotated by a motor 30' (e.g., an air motor or one of another type) of which the shaft is coupled to 3 the tube by a conventional insulating sleeve coupling 31 (FIG. 1) connected to the right hand end of rod 21 at a point to the right of where rod 21 is shown rightwardly broken away in FIG. 3.

The service assembly 18 (FIG. 1) includes a standard tube fitting 35 comprised of a gland nut 62, one or more ferrules (not shown) and a main body 36 through which the rod 21 passes. The body 36 of the fitting is received within and coupled by solder 37 to a stainless steel mounting tube 38 coaxial with rod 21 and containing a portion of that rod which extends from fitting end 36 through service assembly 18 and part way into the quartz cylinder 15 (FIG. 2). Except at its left-hand end 40 (which fits closely around rod 21 to provide a support therefor), the inner diameter of tube 38 is greater than the outer diameter of rod 21 to provide between the tube and rod an annular gas passage 39. When nut 62 is tightened, fitting 35 provides both a torque coupling between elements 21 and 38 and a gas seal for the right hand end of passage 39.

At the left-hand end of unit 18, the mounting tube 38 is provided with an outwardly salient radial flange 45 in the left side of which is formed a leftward-facing annular groove 46 encircling the mounting tube proper. The right-hand end of quartz cylinder 15 is received in groove 46 to axially bear against a gasket ring 47 seated in the groove. Another gasket ring 48 encircles the righthand end of the cylinder and is seated in an annular groove 53 formed in an annular coupling plate 49 in registration with and to the left of flange 45. Plate 49 may be drawn towards flange 45 by the tightening of screws 50 (only one shown) passing through unthreaded holes 51 in the plate into threaded holes 52 in the flange.

The mounting tube 38 is journaled within a pair of high-speed radial thrust bearings 55 on the interior of and secured to a metal sleeve 56. Sleeve 56 is received within and axially slidable in the bore 54 of a cylindrical metal housing 57 fastened to insulating members 58 which are in turn fastened to a support frame (not shown) for the described apparatus. The right-hand end of sleeve 56 is of reduced diameter and is contacted by angularly spaced conventional schematically shown swivel pads 59 on the forward ends of push-screws 60 passing through threaded holes 61 in an inwardly salient radial flange 62 formed at the right-hand end of housing 57 and fitting closely around the reduced-diameter right-hand end of sleeve 56 to provide a support therefor.

During preliminary adjustment of the apparatus, nut 62 is loosened to release tube 38 for axial movement relative to rod 21, and the push-screws 60 are then turned to urge plate 59 leftward so as to force sleeve 56 in that direction. When so forced, the sleeve bears against the flange 45 of tube 38 to drive that flange and tube leftward to thereby cause gasket ring 47 to be axially squeezed between the flange and the right-hand end of quartz cylinder 15. Thereafter, nut 62 is tightened to provide the mentioned gas seal at the right-hand end of passage 39 and to lock rod 21 and tube 38 together so that torque transmitted (FIG. 1) from motor 30 through sleeve coupling 31 to rod 21 is in turn transmitted (FIG. 3) from rod 21 to tube 38.

Gasket ring 47 when axially squeezed provides a gastight end closure for the right-hand end of plasma tube 25. After that gas-tight seal has been established, screws 50 are tightened to draw coupling plate 49 towards flange 45 and to thereby cause the gasket ring 48 to be radially squeezed between plate 49 and the outside of quartz cylinder 15. Because of such radial squeezing, the cylinder is gripped firmly by the gasket ring to thereby be rotatably coupled through elements 49, 50, 45, 38, 37 and 36 to the cathode electrode rod 21. In this manner, the cathode of the plasma tube 25 is locked for rotation with the container envelope thereof.

The gaseous medium 16 within the plasma tube may be introduced into or removed from the quartz container 15 through either one of a pair of quick-disconnect gas fittings 65 received in threadedly engaged relation in holes 66 in mounting tube 38. Each fitting 65 provides an inlet to or an outlet from the annular gas passage 39 desired between the inside of tube 38 and the outside of the cathode rod 21. Although that passage is closed at its left-hand end (see FIG. 2), radial holes 67 in the tube 38 permit flow of the gaseous medium through passage 39 in either direction between one of the fittings 65 and the central region of the plasma tube. The two fittings 65 balance each other relative to the axis of rotation of the plasma tube so as to minimize eccentricity in the mass rotated by the motor 30.

An electrical connection is made to cathode 21 through a lead 70 (FIG. 3) terminating at a brush 71 biased by a spring 72 to bear against a slip ring or disc 73 encircling rod 21 to be in direct electrical contact therewith. Disc 73 has a collar 74 and is fixedly mounted on rod 21 by set screws 74a passing through the collar.

The left service assembly 17 (FIG. 1) is not directly coupled to a motor but otherwise is similar to the right service assembly 18. The unit 17 serves to provide a gastight enclosure for the left end of plasma tube 25 and to lock the quartz cylinder 15 to the anode rod 19 so that both elements rotate together. That is, the rotary motion imparted to quartz cylinder 15 by motor 30 is transmitted through that cylinder to the left-hand service unit and then through such unit to the anode 19 so that the anode rotates in angularly fixed relation with the container cylinder.

An electrical connection is made through unit 17 to anode 19 by way of a lead 75 coupled to a brush and slip ring arrangement (not shown) included in unit 17 and similar to the brush and slip ring (FIG. 3) of right assembly 18. The gaseous medium 16 may be introduced into or removed from the interior of the plasma tube 25 by way of quick-disconnect gas fittings 76 (only one shown) provided by unit 17 and duplicating those of the right-hand unit 18.

The leads 70 and 75 are connected to opposite termi nals of an ionizing pulse oscillator 80 and of a DC. ionizing energy source 81 in parallel with the oscillator. As later described in more detail, the components 80 and 81 are utilized sequentially to energize the electrodes of the plasma tube 25.

The interelectrode gap 24 in the plasma tube is encircled by a single-turn theta compression coil surrounding the quartz container cylinder of the tube. Coil 90 is adapted to be energized by a burst of high energy electric current from a stored energy discharge source 91. Source 91 may be either a bank of selectively dischargeable storage capacitors or a high energy pulse storage network, the latter providing a more or less rectangular output pulse. The initiation of the discharge through the theta compression coil is controlled by a high voltage switch 92. As shown (FIG. 1) switch 92 may be an exploding wire switch controlled by a manually actuated trigger circuit 93. When switch 92 is closed, the resulting burst or surge of current through coil 90 develops in interelectrode gap 24 a transient axial magnetic field B(t) of which the direction of the flux lines is positive, e.g., from left to right in FIG. 1.

As an optional feature of the FIG. 1 apparatus, the plasma tube 25 may be surrounded by a pair of DC. magnet coils 95 and 96 coaxial with the plasma tube and each other and of substantially greater inner diameter than the outer diameter of the theta compression coil 90. Coils 95 and 96 are adapted by flow of DC. current through input lead 98, coil-connecting lead 99 and output lead 100 to develop in interelectrode gap 24 an axial D.C. magnetic field B of which the flux lines have a negative direction, e.g., are from right to left in FIG. 1 so as to be in opposition to the flux lines of the transient field B(t). As shown, coil 95 and 96 are disposed on opposite axial sides of the theta compression coil 90 so as to be spaced apart by a radial window 94. Such window permits radial escape to the exterior of coils 95 and 96 of radiation from plasma within cylinder 15 which is subjected to the transient axial magnetic field B(t) To minimize the inductance in the connection between the high energy discharge source 91 and the theta compression coil 90, that connection is in the form of a flat plate transmission line comprised of spaced parallel lower and upper wide transmission plates 105 and 106 separated by an insulating layer 107 provided by, say, polytetrafiuoroethylene sheeting. In practice, plates 105 and 106 extend all the way to source 91. In FIG. 1, however, the rear portions of the plates have been shown broken away (for convenience of illustration) and are schematically represented in the drawing by the conductors 108, 109.

At their forward ends, plates 105 and 106 are electrically coupled to theta compression coil 90* by a curved shell structure which is specially shaped to fit inside the D.C. magnetic coils 95, 96 while simultaneously providing a low-resistance, low-inductance connection to the compression coil. To describe that structure, forward of an edge 110, the lower transmission plate 105 reduces in width to a flat right-hand arm 111 extending beyond edge 110 past D.C. magnet coil 96 to a front end slightly forward of the axis of the plasma tube 25. At its front end, arm 111 is conductively joined with the lower portion of a vertical conductive strip 112 (FIG. 4) upstanding from the arm and extending axially from the right-hand edge of the arm to point 113. Conductively joined to a section of strip 112 to the right of arm 111 is a semi-cylindrical conductor shell 114 of which the chord line is perpendicular to arm 111. The undersides of the joints of arm 111 and shell 114- to strip 112 are rendered rounded by fairing 115 to thereby reduce corona leakage from those joints.

Upper transmission plate 106 is symmetrical with lower plate 105 in that the upper plate likewise reduces in width at a forward edge 120 to a flat left-hand arm 121 extending beyond that edge and past and to the left of coil 95 to a front end disposed slightly short of the axis of the plasma tube. The forward end of arm 120 is joined to a vertical conductive strip 122 parallel to and spaced from strip 112 and extending axially from the left-hand edge of arm 122 to the shown point 123. Conductively joined to a section of strip 122 to the right of arm 121 is a semicylindrical shell 124 axially co-extensive with (and concentric with) and spaced radially inward of the previously described shell 114. The joints of arm 121 and of shell 124 to the strip 122 are rounded off as shown (FIG. 4) to minimize corona leakage.

Theta compression coil 90 is in the form of a split ring comprised of semicircular halves 130 and 131 clamped together at the top by a screw 132 and separated at the bottom by a small space. The two ring halves 130 and 131 are conductively joined to axially central portions of the upper ends of, respectively, the strip 112 and the strip 122. Insulating material (e.g., polytetrafluoroet-hylene sheeting) fills the spaces between the ring halves 130 and 131, the vertical conductive strips 112, 122 and the conductive shells 114, 124. The upper forward edges 133 and 134 of those shells are rounded as shown (FIG. 4) to minimize corona leakage from the salient upper end of the shell configuration.

Operation The operation of the described system is initiated by actuating motor 30 to spin the plasma tube 25. The full speed rate of spin should be at least 3X10 r.p.m. and preferably r.p.m. or greater. The latter spin rate can be achieved without difficulty by the use of an appropriate motor.

When the plasma tube is so spun, the rotary motions of its container cylinder and the electrodes therewithin are imparted to the contained gaseous medium 16. In this manner, the gaseous fluid of the medium is caused to mechanically spin or rotate about the axis of rotation of the plasma tube at about the same spin rate as that characterizing the tube itself.

After tube 25 is spinning at full speed, the pulse oscillator is actuated to strike an are through the medium 16 in the gap 24 in cylinder 15 bet-ween the electrodes 20 and 22. Oscillator device 80 may be, for example, a 40 kv. spark gap oscillator with an ascillation frequency of 2 mc. When the arc has been struck, it is maintained by the switching on of the D.C. voltage generator -81 of which exemplary operating parameters are: an open circuit output voltage of 320 volts or greater, and (depending on the circuit characteristics of the plasma tube) an output current of from 100 to 1,000 amperes productive of a voltage drop of from 30 to several hundred volts between electrodes 20 and 22. Once the generator device 81 has assumed the load of maintaining the arc, the oscillator 80 is turned off.

The passage of the electric arc through the medium preionizes that medium to create therein a column of plasma referred to herein as the arc plasma column. Because of the high current of the arc and the high density of the medium, the pre-ionization electron density n of the plasma is high, e.g., at least IO /cm. and preferably 5 l0 /cm. to 5 l0 /cm. A pre-ionization electron density of at least 10 cm. is desirable in the described apparatus because the power in joules/m. radiated per unit surface area of radiating region is dependent on such electron density. The pre-ionization electron density of the plasma can be increased by increasing the pressure of the gaseous medium or by seeding it with particles in the gaseous phase of an easily ionizable chemical element. Limitations on increasing the electron density by increasing the pressure are (1) the electron density does not increase in proportion to the pressure, and (2) increased pressure results in a decreased mean free path for the electrons to thereby decrease the mean photon energy level of the emitted radiation.

The mechanical spinning of the plasma tube 25 and of the medium therein produces a corresponding mechanical spinning of the arc plasma column generated in the gap 24 between the electrodes 20 and 22. Such spinning of the column has the following useful effect. The heat generated in the medium by the passage therethrough of the arc creates convection currents in the fluid medium, and those currents act over the length of the arc plasma column to tend to impart to it a buoyancy distortion. That is, if the arc plasma column were not spinning, the convection currents would produce over the length of the column local random radial displacements in the form of the perturbations or kinks similar to those observed in, for example, an are produced in the atmosphere between widely spaced electrodes.

We have discovered, however, that, by mechanically spinning the arc plasma column, there is created a pressure gradient effect by which the column is stabilized in relation to the axis of spacing of the electrodes by which the column is produced. By virtue of such effect, the random radial perturbations of the column are eliminated or rendered inconsequential, and the column itself is caused to assume a stable position concentric with such axis. As later explained in more detail, the stabilization is directly proportional to the density of the medium and to, also, the square of the angular frequency of fluid rotation.

Besides being spun, the arc plasma column is subjected to the D.C. axial magnetic field B generated by passage of D.C. current through coils and 96. Those coils may be energized either before or after establishment of the arc plasma column.

The pre-ionized, spin stabilized arc plasma column is then induced to emit radiation by subjecting it to a magnetic compression effect. The compression effect is produced by actuating trigger circuit 93 to close exploding wire switch 92 to cause the source 91 to discharge a burst of current through the theta compression coil 90. An adequate strength discharge through the coil is provided when source 91 is pre-charged to a voltage value of from 120 kv. to 240 kv.

Because of the low inductance of the single turn of the theta compression coil and because, further, of the low inductance of the parallel plate transmission line 105, 106 and of the curved shell structure by which that line is connected to the compression coil, the LC system formed by source 91, coil 90 and the coupling therebetween is characterized by a high ringing frequency, e.g., at least 300 kc. and preferably 500 kc. or greater. A high value of ringing frequency is desirable for reasons later explained.

The burst of current through coil 90 produces in gap 24 in cylinder 15 a transient axial magnetic field B(t) opposed in direction to the DC. field B The B(t) flux lines do not instantaneously penetrate to the radially interior region of gap 24 to there be admixed with the B flux lines. Instead, the B(t) field forms around the B field a magnetic sheath which implodes at high velocity radially inwards towards the axis of the arc plasma column. The velocity of inward movement of the sheath is preferably on the order of "/cm.-sec.

The imploding magnetic sheath compresses the plasma by exerting on the ions and electrons therein a magnetic pressure which drives the ions and electrons radially inward, and which causes both types of charged particles to be accelerated within the sheath. Because of their large difference in mass, the respective trajectories of the ions and of the electrons are quite different. The heavy mass ions are, relatively speaking, deflected very little by the magnetic pressure and, therefore, move radially inward with only an inconsequential component of motion in the angular or theta direction. The much lighter electrons are, on the other hand, vigorously deflected to describe sweeping arcs in the theta direction.

The difference in the ion and in the electron trajectories increases in the sheath region the mean ion-electron spacing to thereby create in the sheath a net space charge separation and a resulting radial electric field E withiri the sheath. During the early time of application of the compression effect, such radial field is strong enough to exceed in volts/meter the electric field E, which is induced in the angular or theta direction in the region within coil 90 by the changing magnetic field B(t), and which is directed to oppose the fiow of the discharge current through the compression coil 90.

The radial electric field E serves to create across the thickness of the sheath a potential V which is equal to E multiplied by the sheath thickness and which accelerates electrons in the sheath to cause emission of radiation from the plasma in the sheath. The maximum photon energy level of the emitted radiation varies directly with V,.. Under the described conditions of operation, the value of V is, for a short period larger enough to produce radiation with a photon energy level of about one kev., e.g., soft X-rays. That the described apparatus is capable of realizing such result is due in an important degree to the use of a medium containing ions of a high mass numher. To wit, a direct relationship exists between the mass number M of the ions in the medium and V in that (as later pointed out in more detail) the strength of the radial field E variesin direct proportion with M, and V varies in direct proportion with E,. Hence, a high mass number M for the ions in the medium implements the emission of radiation of high photon energy level as, for example, soft X-rays.

As a matter of interest, the photon energy level of the emitted radiation is independent of the voltage to which source 91 has been pre-charged, but the power output in joules/meter represented by that radiation is in direct proportion to that voltage. Such power output also varies directly as the 3/2 power of the pre-ionization electron density of the plasma, wherefore, in the described apparatus, a high pre-ionization electron density is provided, i.e., an electron density of 10 electrons/cc. or greater.

A contributory factor to the power (in joules/meter of the emitted radiation is the DC. magnetic field B While the use of that field is optional, its presence broadens the thickness of the magnetic sheath to thereby increase the volume of the region inside the sheath from which radiation is emitted. Thus, by using a B field, the radiation output can be enhanced.

Both the strength of the radial sheath potential V and the character of the radiation vary as a function of the time t after onset of the discharge through the compression coil 90. The potential V falls off rapidly with time so as to drop from, say 7 l0 volts at 10* seconds after onset to a value of, say, only 0.7 volt at 10* seconds after onset. With respect to the character of the radiation, during the first 10 seconds after onset, the sheath electron energy distribution cannot be properly characterized by a temperature but is more nearly monoenergetic, and the emitted radiation is probably predominantly from non-thermal electrons. After 10 seconds, the character of the radiation is believed to change so as to be, first, mostly Bremstrahlung emitted by thermal electrons and, thereafter, mostly residual recombination radiation and line radiation from multiply ionized ions. The entire burst of emitted radiation of interest is produced within a period of substantially less than 1 microsecond.

Because the major portion of the desired emitted radiation occurs within the first 10* seconds after onset, and because the photon flux of the emitted radiation varies directly with the discharge current passing in that period through the theta compression coil 90, it is desirable to maximize the rate of rise within that period of the discharge current. This is done in the described apparatus by minimizing the inductance of the discharge system formed of source 1, coil and the coupling therebetween. Specifically, such inductance is made as small as practical by (1) using a compression coil of only a single turn and otherwise of low inductance, and (2) employing the described low-inductance coupling between the coil 90 and the source 91. Alternative ways of expressing the low inductance of the discharge system are in terms of its ringing frequency and the width of the pass band provided for current from source 91 by coil 90 and its coupling to source 91, both the ringing frequency and the width of such pass band varying inversely with the system inductance. For the purpose of producing a copious photon flux output, we have found that, as a practical matter, the ringing frequency of the discharge system should be at least 300 kc. (and, preferably, 500 kc. or more) and, correspondingly, the width of the described pass band (as measured between zero and the half current point) should be at least 300 kc. (and, preferably, 500 kc. or more).

QUANTITATIVE ASPECTS OF OPERATION The following is a simplified explanation in quantita two terms of the manner in which the described apparatus is believed to operate. While such explanation is deemed to conform with reasonable accuracy to what actually occurs, the phenomena and mechanisms by which the apparatus is caused to emit radiation may, in fact, be more complicated than the explanation suggests. Accordingly, we do not wish or intend to be limited or bound in respect to our invention by the explanation given herein of its believed mode of operation.

I. Spin stabilization As previously described, the arc plasma column is stabilized by mechanically spinning the pre-ionized fluid of the medium 16 about the axis for the column. The stabilizing provided by the spinning can be adduced mathematically from the following equations of motion of individual ions and electrons in a frame of reference rotating with the fluid velocity.

9 The Hamiltonian for axisymmetric motion in cylindrical coordinates r and is:

ai a 2m 2mr where p is the momentum, In is the particle mass and Y is the potential energy of the particle. The equation for p is then:

but

LY z 0 and:

where F is the radial force on the particle and w is the angular frequency of fluid rotation. Since the force F can be written as 6Y/6 we can plot Y(r) vs. r and we obtain the curve 150 shown in FIG. 5 wherein the Z axis is the axis of spin 0f--th e. plasma tube 25, and the dotted lines 151 indicate the innerv wall surface of the container cylinder 15. In FIG. 5 the ordinate and abscissa are, respectively, the value of Y(r) and the radial distance r from the Z axis. Curve 150 shows that Y(r) varies as the 4th power of r. As demonstrated by curve 150, the pressure gradient caused by spinning of the medium serves to produce a potential energy well.

If we assume a conservative system, the sum of the kinetic energy and potential energy of a given particle is constant. When, as in FIG. 5, E Y/E for the article is positive at the equilibrium positio (the bottom of the potential energy well), a displacement of the particle in either direction increases its potential energy and hence decreases its kinetic energy. Hence, each particle (and, also, the column as a whole) is in static equilibrium (zero kinetic energy) and cannot climb out of the well without external help.

As indicated by the foregoing equations, the magnitude of the stabilizing effect is directly proportional to the density of the fluid medium and, also, to the square of the angular frequency of fiuid rotation.

II. Dynamic sheath effects In the following analysis, the properties of the magnetic sheath are reviewed and are then related to the properties of the plasma compressed by the sheath.

(1) Sheath velocity.FIG. 6 shows schematically the theta compression coil 90 and associated components and, also, the arc plasma column and sheath.

Referring to FIG. 6, at the onset of an idealized theta compression, the flow of current through coil 90 is opposed by an induced voltage E, which is the only voltage present at that instant. The voltage on the capacitor (V,,) must at all times be equal to the voltage drop around the coil 90 if the switch 92, transmission line 105, 106 and coupling 111, 121 (and associated unshown coupling components) have no inductance. Thus,

where B(t) is the magnetic flux density associated'with the current in the load coil 90. If B(t) is assumed constant in amplitude and direction but varies as a sinusoidal function of time (i.e., is instantaneously equal to B sin to t) then:

10 and 1 an: Va )4 E 2 dt-R 21rR R (4) When a pre-ionized medium is present within coil 90, the discharge therethrough causes the formation of a sheath of thickness 25 where 5: (r-a), and:

But da/dt is the sheath implosion velocity (v From FIG. 6, we see that (ra) equals 6. If the sheath is thin, r can be approximated as:

and we obtain the approximation -Z l E0222 (27)5T2QB UBB (l) or E0=6wB+v B(t) in rationalized M.K.S. units.

In a typical case of interest, the first term is less than one part in a thousand of the second term, and a good approximation of Equation 9 is volts The upper limit of v can be determined by imposing Equation 4 where E, is the vacuum value at r. The expression for the sheath velocity is then:

1' V E0(7')E s TV *zm nm (11b) FIGS. 7 and 8 illustrate the plasma sheath parameters and the magnetic field which are shown in the drawings both without (FIG. 7) and with (FIG. 8) the bias field B0- As indicated by FIGS. 7 and 8, the sheath travels with velocity v in the r direction corresponding to the inward radial direction. In FIG. 7, the magnetic field in the Z direction (axial) is reduced from its driving value B(t) to zero over the sheath thickness 26. The essential property of such a sheath is that the electrons in the plasma feel an induced electric field E, as they are overtaken by the sheath, and, accordingly, they are accelerated in the 0 direction which is transverse to the direction of propagation of the sheath. The reverse field B in FIG. 8 provides a mechanism for creating a thicker sheath than 3 would normally occur in its absence.

11 (2) Sheath thickness.From the previous considerations the sheath thickness can be deduced as follows.

Referring to FIG. 9, by Ampere law:

Substituting into Equation 13 Equation 10 and noting that we obtain:

B(t)|-B 2e n,,6 ga fi B(t) e mC C where o is the plasma frequency. 'Hence from (14) we obtain:

C B(t) +B 1/2 -wp meters If B B(t) there is an appreciable enhancement of the photon flux of the emitted radiation, the enhancement being a condition which occurs in time near t=0 on the first quarter cycle of the discharge.

(3) Radial electric field.In the previous discussion the effects of the ion population have not been considered. The effect of the ions is twofold. First, the massive ions by their inertia balance the magnetic pressure B2/2/.L0 driving the sheath. That is, the advancing sheath is like a massive piston which projects the ions and electrons inward with a velocity 2v (relative to the laboratory frame of reference) and, concomitantly sweeps them up. Momentum balance then requires:

JT?=1w,,(2Mv,)=2,,Mv,, The second effect of the ions is the generation of a net space charge in the sheath which gives rise to the electrostatic field E in the radial direction. That field is generated because there is a large difference in the trajectory of the electrons and the ions in the sheath due to the difference in masses. The ions are essentially not deflected by the magnetic field and therefore move in the radial direction. The electrons, however, describe sweeping arcs in the 0 direction. The resulting charge separation produces the electric field E which can be expressed An order of magnitude estimate of E /B can be obtained by a simple argument which yields:

If we assume an ion mas number of -130 (cesium or xenon) and an arc column radius of 1 cm., then:

Under the assumed condition, V varies with time t in accordance with the following tabulation:

1 sec.: V volts 10- 7x10 10- 7 10 10- 7 10 10- 7 10 10* 0.7

From tabulation 22, we see that the sheath voltage is sufiiciently high to accelerate electrons to -1 kev. during the first 10- seconds after closure of the switch 92 in FIG. 6.

(4) Photon energy level.-Referring to Equation 20, the voltage V sets the maximum photon energy level of the emitted radiation in the sense that, if an electron is accelerated through that full voltage, the resulting radiation will have the highest photon energy level attainable under the given operating conditions. In practice, the maximum photon energy level is not attained for every electron accelerated by V because a substantial percentage of the accelerated electrons undergo a collision before being accelerated through the full voltage V Thus, the attained mean photon energy is less than the maximum attainable photon energy level. At the same time, with other conditions being kept constant, the mean photon energy level increases with an increase in the maximum photon energy level.

Equation 20 further indicates that the voltage V varies directly with the mass number M of the ions in the compressed plasma. Hence, increasing the mass number of those ions increases the maximum attainable photon energy level of the emitted radiation and, also (with other conditions being kept constant), its mean photon energy level.

The latter level is a partial function of the mean free path of the electrons accelerated by V,. If the pressure of the medium is low so that the mean free path is long (e.g., substantially greater than the sheath thickness), then the mean photon energy level approaches the maximum photon energy level. If, on the other hand, the mean free path is short (e.g., substantially less than the sheath thickness), then the mean photon energy level is lowered relative to the maximum photon energy level.

Thus, while the maximum photon energy level is set by the mass number M of the ions in the medium, the mean photon energy level can be progressively reduced from the maximum attainable value by increasing the pressure of the medium.

By utilizing a medium of high mass number (e.g., pure xenon) and of low pressure (e.g., microns Hg), it is feasible with the described apparatus to produce hard X-rays. Of more interest, however, it is the production of radiation in the wavelength region embracing soft X- rays and ultraviolet rays of short wavelength because it has been difiicult heretofore to artificially produce radiation in that wavelength region. We have found that radiation in such preferred region can readily be provided with the described apparatus when operating in the described manner by appropriate selection in combination of a suitably high mass number M for the medium and of a suitable value for the pressure of the medium. Thus, for example, when the plasma tube is charged with pure xenon (for which M is about at a pressure of about one atmosphere absolute, the described apparatus is adapted to produce radiation having a mean photon energy level such that the emitted radiation contains a usefully large percentage of soft X-rays of about 1 kev.

The product of the mass number M of the medium and of its pressure is the density of the medium. For the reasons just set out, there is a correlation between the density of the medium and the suitability thereof to act as a source of radiation in the mentioned preferred wave-length region. That is, in order to produce radiation containing a usefully large percentage of soft X-rays or of ultraviolet rays of short wavelength, the density of the medium should be in the range from 0.02 kg./meter to 2.0 kg./meter (5) Photon flux output.With the described apparatus, a radiated power value on the order of several hundred joules is obtainable. For a given mean photon energy level, the radiated power varies directly with the emitted photon flux of which the value is dependent on the following factors.

First, because all or most of the energy represented by the photon flux is derived from the compression field B(t) and because the strength of B(t) varies with the voltage V to which source 91 has been-precharged, increasing V increases the photon flux.

Second, the photon flux output varies directly as the 3/2 power of the electron density characterizing the plasma. A high electron density is realizable as follows: The field B tends to increase the electron density of the arc plasma column by inhibiting diffusion of electrons out of the column. Further, the electron density of the column can be enriched by seeding the parent gaseous medium with particles in the gaseous phase of a chemical element having a low ionizing potential, e.g., cesium. While the seeding particles have little effect on M and thus upon the maximum or mean photon energy of the emitted radiation, the particles do, upon ionization, act as donors of electrons to the plasma to increase its electron density and, accordingly, the emitted photon fiux.

Still further, the electron density can be increased by increasing the pressure of the medium. The last-named mode of increasing electron density is, however, limited in effectiveness by the considerations that, (a) the electron density does not increase in direct proportion with the pressure, (b) an increase in pressure results in a decrease in means photon energy level relative to the maximum attainable photon energy level.

Third, besides increasing the electron density, the field B increases the photon flux by increasing the thickness of the sheath (see FIG. 8) to thereby increase the volume of the region of the plasma from which radiation is emitted.

Fourth, the greater part of the radiation is, as described, emitted in the first 10* seconds after onset of the discharge pulse through compression coil 90. Hence, the photon flux output is increased by maximizing in that short period between 1 equals zero and t equals 10- the conditions which implement the production of a large flux. Specifically, in the described apparatus the field B(t) produced within that period is maximized in strength by minimizing the inductance of the discharge system comprised of source 91, coil 90 and the coupling therebetween (i.e., maximizing the ringing frequency of such system). Moreover, that the plasma will have a high electron density in that period is assured by ionizing the medium to have a high electron density before the compression field B(t) is applied. That is, by passing of the are between the electrodes 20 and 22 for a suitably long interval before onset of the compression field, the medium 16 within interelectrode gap 24 is pre-ionized to present a plasma of high electron density to the compression field when generated. As stated, the pro-ionization electron density produced in the described apparatus is at least 10 electrons/cc. and preferably is larger, i.e., X electrons/ cc. to 5 X 10 electrons/ cc.

Composition and properties of the medium Although medium 16 has been described above as being composed of pure xenon, the medium is not necessarily limited to such composition. Alternatively, the medium may be composed of a pure monatomic gas other than xenon (e.g., krypton, radon), a mixture of monatomic gases, metallic vapors (e.g., cesium vapor, mercury vapor), or a mixture of one or more monatomic gases and one or more metallic vapors. While diatomic or other molecular gases are not necessarily wholly unsuitable for use as or in the medium, they have the disadvantage that much of the energy of the arc is absorbed in effecting molecular dissociation and is, therefore, not available for producing a high electron density in the plasma.

The appropriate pressure for the medium depends upon its composition and upon the spectral energy content re quired for the radiation from a medium of that composition. In general, however, the pressure of the medium is greater by a factor of at least 10 than that utilized in prior art experimental pinch devices aiming at the attainment of a self-sustaining nuclear fusion reaction.

As mentioned, heretofore, the medium may be in the form of a parent gas seeded with a material which is in the gaseous or vapor phase, and which is more easily ionizable than the parent gas. Preferably, the parent gas is, as described above, a pure monatomic gas or a mixture of monatomic gases. The seeding material is preferably one of the alkali metals (e.g., potassium, sodium, etc.) because they are the most easily ionized of all the chemical elements. Cesium is particularly suitable as a seeding material because of its very low ionizing potential.

Seeding may be accomplished by vaporizing the seeding material into the parent gas and by heating the resulting mixture so as to maintain the vapor pressure of the seeding material at a significant value. Usually the concentration of seeding material in the medium is too low to have a substantial effect on its mass number M (which, therefore, is effectively determined entirely by the com position of the parent gas). Even at such low concentration, however, the seeding material may enhance considerably the electron density of the plasma generated in the medium.

The above-described embodiments being exemplary only, it is to be understood that additions thereto, modifications thereof and omissions therefrom can be made without departing from the spirit of the invention, and that, therefore, the invention comprehends embodiments differing in form and/or detail from those specifically disclosed. Accordingly, the invention is not to be considered as limited save as is consonant with the recitals of the following claims.

We claim:

1. The method of producing radiation comprising, passing externally generated electric current through electrodes spaced apart in a gaseous medium having an initial density of at least 0.02 kilogram/meter and through the interelectrode gap in said medium so as to pre-ionize said medium to an electron-density of at least 10 electrons/ cc. by generating in said gap a plasma column in the form of an electric thermal arc, and thereafter compressing said pre-ionized medium by a magnetic field so as to induce emission from said medium of radiation Within the Wavelength interval of the electromagnetic spectrum occupied by soft X-rays and ultra-violet rays.

2. The method as in claim 1 in which said medium is comprised of a monatomic gas.

3. The method as in claim 2 in which said monatomic gas is xenon.

4. The method as in claim 1 in which said medium is comprised of a parent gas seeded with ionizable particles in the gaseous phase.

5. The method as in claim 4 in which said particles are more easily ionized than said parent gas.

6. The method as in claim 1 in which said magnetic field is developed by flow of electric current in the theta direction relative to an axis through said medium.

7. The method as in claim I in which said compression field acts in opposition in said medium to a supplemental static magnetic field previously applied to said medium.

8. The method comprising, introducing an ionizable gaseous medium into a rotatable enclosure therefor, gen erating an electrical discharge within said enclosed gaseous medium to produce a body of plasma therein, and rotating said enclosure to impart to said enclosed 15 medium a mechanical rotation around an axis of rotation for said enclosure to positionally stabilize said body of plasma relative to said axis, said body of plasma being urged by a pressure gradient effect derived from said rotation of said medium to stay in coaxial relation with said axis.

9. The method comprising, introducing a gaseous medium into a container therefor, energizing spaced electrodes within said container to produce an arc plasma column in said medium between said electrodes, and spinning said container and electrodes around the axis of spacing of said electrodes to positionally stabilize said column relative to said axis.

10. The method comprising, pre-ionizing a gaseous medium to produce a plasma column therein, imparting to said pro-ionized medium a mechanical rotation around an axis for said column to positionally stabilize said column relative to said axis, and compressing said stabilized plasma column by a magnetic field so as to induce emission of radiation from said medium.

11. The method as in claim in which said medium has an initial density of at least 0.02 kilogram/meter 12. Apparatus comprising, container means for a gaseous medium, first and second electrode means spaced along an axis in said container means and responsive to energization to develop an arc plasma column in said medium, and means to rotate said container means and electrode means around said axis so as to positionally stabilize said column relative to said axis.

13. Radiation-generating apparatus comprising, container means for a gaseous medium, first and second electrode means spaced along an axis in said container means and responsive to energization to develop an arc plasma column in said medium, means to rotate said container means and electrode means around said axis so as to positionally stabilize said column relative to said axis, and means to compress said column by a magnetic field productive of emission of radiation from plasma in said column.

14. Apparatus as in claim 13 in which said compressing means is comprised of a single turn theta compressing coil.

15. Apparatus as in claim 13 further comprising means to apply to said stabilized column a supplemental magnetic field opposing said compression field.

16. Radiation-generating apparatus comprising, container means for a gaseous medium and characterized by a central axis, anode and cathode electrodes spaced in said container means along and coaxial with said axis and responsive to energization to develop an arc plasma column in said medium between said electrodes, means to mechanically rotate said container means and electrodes around said axis so as to positionally stabilize said column relative to said axis, and a theta compression coil disposed axially between said electrodes and around said container 16 means, said coil being responsive to a discharge therethrough of electrical energy to apply to said stabilized column a magnetic compression field by which the plasma in said column is induced to emit radiation.

17. Radiation generating apparatus comprising; container means filled with a gaseous medium having an initial density of at least 0.02 kilogram/meter first and second electrode means spaced from each other in said container means by an interelectrode gap provided in said medium, electrical energy source means coupled in circuit with said two electrode means and said interelectrode gap to produce in said gap a plasma column which is in the form of an electric arc and which pre-ionizes said medium, and means to compress said pre-ionized medium by a magnetic field so as to induce emission from said medium of radiation within the wavelength interval of the electromagnetic spectrum occupied by soft X-rays and ultraviolet rays.

18. The method as in claim '8 in which said body of plasma is urged by said pressure gradient effect and magnetic field to stay out of contact with the inside of said enclosure.

19. A system comprising, a rotatable enclosure for a gaseous medium, means to introduce said medium into said enclosure, ionizing means to produce in such enclosed medium a body of plasma, and means to rotate said enclosure around an axis of rotation therefor to impart to said enclosed medium a mechanical rotation so as to produce in said medium and relative to said axis a radial pressure gradient for which the direction of increasing pressure is the radially outward direction, and which positionally stabilizes said body of plasma by urging such body to stay in coaxial relation with said axis, said means coacting with a source of magnetic flux to further urge said body of plasma to stay out of contact with the inside of said enclosure.

References Cited UNITED STATES PATENTS 2,997,436 8/19611 Little et a1. 313-161 X 3,006,835 10/1961 Quinn et a1. 313-161 X 3,089,831 5/1963 Kolb 313-161 X 3,290,219 12/1966 Hurwitz 313-161 X 3,292,028 12/1966 Van Ornum 315-1111 X 3,340,415 9/1967 De Ruiter et a1. 3151|l1 X JAMES W. LAWRENCE, Primary Examiner.

R. F. HOSSFELD, Assistant Examiner.

US. Cl. X.R. 

